It is demanded for the display surfaces of image display devices such as liquid crystal displays (LCDs) and cathode ray tube display devices (CRTs) to reduce the reflection of rays emitted from external light sources such as a fluorescent lamp in order to improve the visibility of these display surfaces.
A phenomenon has been known that reflectance is made smaller by coating the surface of a transparent material with a transparent layer having a small refractive index. It is possible to improve visibility by arranging an anti-reflecting layer utilizing such a phenomenon on the display surface of an image display device. The anti-reflecting layer has a monolayer structure in which a low-refractive index layer having a small refractive index is disposed on a display surface or a multilayer structure in which one or plural layers having middle to high refractive indexes are disposed on a display surface and a low-refractive index layer for decreasing the refractive index of the outermost surface is disposed on these middle to high refractive index layers to produce a higher anti-reflecting effect.
The monolayer type anti-reflecting layer is simpler than the multilayer type and is superior in productivity and cost performance. On the other hand, the multilayer type anti-reflecting layer can be improved in anti-reflecting performance by combining layer structures and is therefore improved in performance more easily than the monolayer type.
Methods of forming a low-refractive index layer included in such an anti-reflecting layer are roughly classified into a vapor phase method and a coating method in general. The vapor phase method includes physical methods such as a vacuum deposition method and sputtering method and chemical methods such as a CVD method. The coating methods include a roll coating method, gravure coating method, slide coating method, spraying method, dipping method and screen printing method.
In the case of using the vapor phase method, a high functional and high quality transparent thin layer can be formed. However, in this method, it is necessary to control the minute atmosphere in a high vacuum system, and also, a specific heating device or an ion generating and accelerating device is required, giving rise to the problem that production equipment is complicated and large-sized, entailing high production costs. Also, in the case of using the vapor phase method, it is difficult to enlarge the area of a transparent thin layer and to form a transparent thin layer in a uniform layer thickness on the surface of a film or the like having a complicated shape.
On the other hand, in the case of using the spraying method among the coating methods, the utility efficiency of a coating liquid is low, posing the problem such as a difficulty in controlling filming conditions. In the case of using the roll coating method, gravure coating method, slide coating method, dipping method or screen printing method, the utility efficiency of layer raw materials is high and there is an advantage in mass-production and equipment cost. However, there is the problem that a transparent thin layer obtained by the coating method is usually more deteriorated in functions and qualities than that obtained by the vapor phase method.
As a coating method, a method is known in which a coating liquid containing a polymer (fluorine-containing polymer) having a fluorine atom in its molecular structure is applied to the surface of a support and dried or a coating liquid containing a monomer (fluorine-containing monomer) having a fluorine atom in its molecular structure is applied to the surface of a support and dried, and then cured by UV radiation to form a low-refractive index layer. A coating layer comprising a binder containing fluorine has a low refractive index and the refractive index is more reduced with an increase in the fluorine content. Also, when the fluorine content in the coating layer is increased, such an effect that the coating layer is resistant to the sticking of soils, improving anti-stain characteristics is produced. However, because the intermolecular force of fluorine itself is small, a molecule containing a fluorine atom tends to be softened, posing the problem that if the fluorine content in a coating layer is high, the hardness and strength of the coating layer are decreased.
Because many low-refractive index layers are principally formed on the outermost surface or in the vicinity of the surface of an anti-reflecting layer, these layers basically tend to be suffered from the attacks of some materials by a contact to these materials, or collision or friction with these materials. If the fluorine content is too increased to raise the refractive index of the coating layer, this brings about a remarkable drop in hardness, so that the layer is easily damaged and there is therefore the case where the layer is damaged even if it is only rubbed hardly to wipe dusts and stains.
Also, when the low-refractive index layer is disposed as an intermediate layer though it is disposed in the vicinity of the surface, it is somewhat less damaged by attacks from the outside. However, if the fluorine content is too increased to raise the refractive index of the coating layer, this causes a reduction in strength, with the result that peeling at the interface between the low-refractive index layer and a layer adjacent to the low-refractive index layer is easily caused by the concentration of stress.
A low-refractive index layer is formed by mixing a binder having a high fluorine content with a binder containing no fluorine and having high hardness or strength, whereby the hardness and strength of the low-refractive index layer can be improved. In this case, however, the proportion occupied by fluorine to be contained in the whole of the low-refractive index layer is low and therefore the effect of the fluorine-containing binder on a reduction in refractive index is impaired, bringing about the results that the refractive index can be insufficiently reduced.
Disclosed in the publication of JP-A No. 11-64601 is a low-refractive index layer having a very fine porous structure prepared by forming micro voids having an average diameter of 200 μm or less in a coating layer comprising of a fluorine-containing polymer. The low-refractive index layer disclosed in this publication ensures that the refractive index of the coating layer can be made to be close to that of air (namely, refractive index: 1) by forming a large number of micro voids in the coating layer and it is therefore possible to reduce the refractive index even if the fluorine content in the fluorine-containing polymer is not raised. Also in this case, however, if the number of micro voids is too increased to reduce the refractive index of the coating layer, this causes reduction in the hardness and strength of the coating layer similarly to the case of raising the fluorine content.
The present invention has been attained in view of the above situation, and a first object of the present invention is to provide a coating composition capable of forming a fluorine-containing coating layer which has a low refractive index, high hardness and high adhesion.
Also, a second object of the present invention is to provide a coating layer having a low refractive index, high hardness, high adhesiveness, high transparency and is also superior in mass-productivity as the coating layer can be formed by a coating method, by using the coating composition which can attain the above first object.
Also, a third object of the present invention is to provide a coating layer used preferably as an optical thin layer which has a low-refractive index, high hardness, high adhesion and high transparency, is superior in mass-productivity and of which a low refractive index is demanded, particularly, as a low-refractive index layer of an anti-reflecting layer, by using the coating composition capable of attaining the above first object.
Also, a fourth object of the present invention is to provide an anti-reflecting layer having an optical thin layer comprising the coating layer which can attain the above second and third objects and an anti-reflecting film and an image display device to which such an anti-reflecting layer is applied.
The present invention attains at least one of these objects.